Extracellular glutamate is known to be excessively elevated both ictally and interictally in the epileptic human hippocampus. However, the source and significance to epileptogenesis of these high levels of glutamate remain a matter of debate. The overall goal of this proposal is to better understand the cellular and metabolic mechanisms that underlie the elevations of glutamate seen in the hippocampus of patients with mesial temporal sclerosis (MTS). There is compelling evidence for both neuronal and glial dysfunction n MTS and much of the work to date has focused on changes in either neuronal or glial function in isolated preparations. We postulate that the "neuronal-glial unit" is impaired in MTS such that the dysfunctional glia Droduce the bulk of the elevated interictal glutamate. Moreover, while both neuronal and glial release may contribute to ictal elevation, we hypothesize that the glial component is very substantial. We propose to test this hypothesis using a wide array of techniques aimed at understanding the disposition of glutamate in the epileptogenic human temporal lobe. We will couple these studies to the intracranial monitoring used to dentify epileptogenic regions in patients with medically intractable epilepsy. In Specific Aim 1, we will use HPLC to measure extracellular glutamate and glutamine in microdialysate both interictally and following stimulation using microdialysis probes attached to the depth electrodes. Stimulation will be done to simulate focal seizure activity. In Specific Aim 2, we will infuse 13C acetate, which is exclusively taken up by glia, and will use mass spectrometry to measure the fractional enrichment of 13C in glutamate and glutamine in the microdialysate at baseline and following stimulation to determine the source(s) of extracellular glutamate under these different conditions. For both these studies, we will perform the analyses in probes placed in the hippocampus and in a non-epileptogenic cortical region. Glutamate uptake and efflux will be measured in Specific Aim 3 using resected cortical and hippocampal tissue. In addition physiological studies on the effects of elevated glial glutamate will be done as part of this aim. Possible mechanisms for glial release will be addressed as well. Finally, in Specific Aim 4, the levels of the enzymes and transporters most relevant to glutamate metabolism in glia and neurons will be measured by Western blots using resected tissue. The data obtained in each Specific Aim will be correlated with quantitative neuronal and glial counts. Relevance: Taken together these data will allow us to develop a better understanding of the origins of the elevated glutamate in MTS, and thus to devise more targeted molecular and cellular therapies to restore glutamate homeostasis and thus decrease excitability and seizures.